TCAM has been proposed in ITU-T SG15 11RV-046 (i.e. hereinafter “proposed TCAM”), the contents of which are incorporated by reference herein in their entirety, as a way to seamlessly adapt to quick variations of noise level that cannot be tracked with traditional seamless rate adaptation (SRA) or other on-line reconfiguration (OLR) schemes. The proposed TCAM scheme allows for a receiver controlled adaptive modulation (RCAM or RCM) mode to be used during lower and more stable noise level conditions. In the proposed TCAM, a hierarchical quadrature amplitude modulation (HQAM) scheme is used in both TCAM and RCAM modes, using bit loadings calculated by the receiver. The HQAM scheme separates a QAM constellation into “base” and “enhancement” layers and allows for mapping of data to “enhancement” layers in the constellation to be turned off by the transmitter during periods of high noise, thereby preserving adequate noise margin. When the receiver is thereafter able to perform an adequate noise measurement, it re-computes new bit loadings and sends them to the transmitter, which can then resume mapping of data to the “enhancement” layers according to the newly computed bit loadings.
The present inventors have noted that the proposed TCAM does not allow for coding such as inner trellis coded modulation (TCM) to be used, thereby reducing the system capacity in terms of transmitted bit-per-second when line conditions would otherwise permit it to be used (e.g. during RCAM mode). Accordingly, it would be desirable to have a system and method that overcomes these drawbacks, among others.